Screen for photographic purposes, photomechanical printing processes and the like



Feb. 7, 1933. R TR|$T 1,896,567

SCREEN FOR PHOTOGRAPHIC PURPOSES, PHOTOMECHANICAL PRINTING PROCESSES A THE LIKE Filed Oct. 28, 27 3 Sheets-Sheet l III A. R. TRIST. 1,896,567 APHIC PURPOSES, PHOTOMECHANICAL THE LIKE Feb. 7, 1933.

SCREEN FOR PHOTOGR PRINTING PROCESSES AND 3 Sheets-Sheet 2 Filed Oct. 28, 1927 Feb. 7, 1933. R, TR|5T 1,896,567

SCREEN FOR PHOTOGRAPHIC PURPOSES, PHOTOMEGHANICAL PRINTING PROCESSES AND THE LIKE Filed Oct. 28, 192') 3 Sheets-Sheet 3 fliiii ynnlu' il llllll Patented Feb. 1, 1933 UNITED STATES PATENT OFFICE ARTHUR RONALD TRIST, OF LONDON, ENGLAND, ASSIGNOR, BY-MESNE ASSIGNMENTS, TO PANTONE CORPORATION, OF NEW YORK, N. Y., A CORPORATION OF DELAWARE SCREEN FOR PHOTOGRAPHIO PURPOSES, PHOTOMEOHANIGAL PRINTING PROCESSES AND THE LIKE Application filed October 28, 1927, Serial No. 229,314, and in Great Britain November 4, 1926.

This invention relates to improvements in the manufactureof screens for photographic purposes, photomechanical printing =processes and the like said screens being of the type in which the changes in opacity of the different parts are gradual and has for its object the provision of means whereby greater and more reliable control in the gradation of the change of opacity is available thus enabling more accurate gradation of the tones of a picture and a more faithful reproduction of the picture to be obtained than has been possible heretofore.

This invention consists in the arrangement of a light tight chamber; a half tone or ruled screen in half tone relation to a film of photographically sensitive material arranged behind said screen in said chamber; an aperture in said chamber in half tone relation to' said screen, said aperture being similar in shape to the shape of the openings formed by the rulings of the screen employed; a rotatable diaphragm associated with said aperture, the opaque part of said diaphragm being of such shape that it will when rotated give a predetermined rate of change of opacity in the multiple images thereof on the sensitive material; a source of illumination and means for continuously rotating said diaphragm to produce a screen having shaded areas.

In the drawings Figure 1 illustrates a portion of a screen having apositive image formed according to this invention and drawn to an enlarged scale; a

Figure 2 is a detail diaphragm employed in the manufacture-of the screen illustrated in Figure 1;

Figure 3 illustrates a portion of a screen having a negative image formed according tothis. invention and drawn to an enlarged scale;

Figure 4 is a detail View of a rotatable diaphragm employed in the manufacture of the screen illustrated in Figure 3;

Figure 5 illustrates in elevation one form of apparatus for producing the screens illustrated in Figures 1 and 3;

- Figure 6 is a transverse section of amount surface of b.

one part c to substantial photographic viewof a rotatable suitable for use in the arrangement illustrated in Figure 5; i v

Figure 7 is a front elevation of the carrier for the rotatable diaphragm; and

Fig. 8 diagrammatically, illustrates the arrangement of'the various parts during the photographing step in the manufacture of the improved screen.

In Figure 1 of the drawings, a a are comparatively small areas, which, for the sake of simplicity are illustrated as of square shape, said areas being contiguously arranged upon one surface of a transparent base I), in such number as to cover the whole It will be seen that, as is usual in screens having shaded areas, the photographic density of each of the small areas a varies from substantial photographic transparency at opacitv at the other part (1. I I

In Figure 1 the photographically transparent part 0 is located at the centre of each area a whilst in Figure 3 the transparent part 0' is at the margin ofeach area a.

The rate of change of density from centre c to border d of each of the areas a has to be adjusted so that the tones of the printed reproduction bear a definite relationship to thetones of the picture reproduced.

In Figure 3 of the drawings, a a are com-- paratively small areas comparable with areas a a of Figure 1, said areas being, for the sake of simplicity, illustrated as of square shape. The areas a a are contiguously arranged upon one surface of atransparent base 6 in such number'as to cover completely the whole surface of b.

As arranged in Figure 3, the photographic density of each of the small areas a yaries from substantially photographic opacity at a the centre d to substantially photographic transparency at the outer part 0 Although .the contiguously arranged areas a have been illustrated in Figures 1 and 3 as of square shape, such areas may be of any geometrically similar shape provided that the whole of the surface of the base I) can be covered with such areas.

In the production of a screen by meansof the invention and having square areas a ruled half tone or any similar screen having a line to space ratio of 1:1 may be used as hereinafter explained.

Careful experiment has shown that by means of a rotating diaphragm it is possible to exercise that control of density gradation which is essential to success.

Figure 5 shows apparatus suitable for producing a screen of the type illustrated in Figures 1 and 3 and Figures 2, 4, 6 and 7 illustrate the operative elements in detail. In Figures 5 and 6 a frame 6 is formed integrally with a mount 7 and attached to the front board 7 of the light tight chamber,-

said frame 6 being provided with bearings 71. and aaspindle z carrying a friction roller 3' and a driving pulley k.

A carrier Z is adapted to be located within the frame e said carrier having arotatable part m which is mounted upon rings of balls at and n in order that the said part m may rotate with the minimum of friction and vibration. The part m has an aperture 0 and a seating 7 within which a diaphragm 72 (such as that illustrated in Figure 2) may be disposed, said diaphragm 39 being maintained' in place in the seating p by means of suitable spring clips, not shown.

The carrier Z is also provided with a nonrotatable diaphragm having an aperture 1- geometrically similar in shape with the shape of the areas, for example, if the boundary of each of the small areas is to be in the shape of a square, as illustrated in Figure 1, then the aperture 1" will be square in shape;

The part m is provided with a grooved edge for engagement with the friction roller 7'which is rotated from any suitable source of motion; as illustrated in Figure 5 a clockwork motor 8 having a pulley t transmits motion to the pulley by means of the belt at.

The diaphragm p as illustrated in Figure 2 is formed of transparent material the patch 1) comprising a plurality of portions of circumferential bands, zones or areas, the circumferential length of each band or the like being adiusted to suit the desired rate of change of density across each of the areas a of Figure 1.

The diaphragm p as illustrated in Figure 4 is formed of opaque material and'has an aperture 9 therein shaped to produce the shaded areas a of Figure 3, that is to say, if the rate of change of density across each area a of Figure 3 is similar to the rate of change of density across each area a of Figure 1 then the aperture r in the diaphragm 12 will be similarly shaped to the patch a in the diaphragm p During the preparation of a screen with the improved apparatus a ruled or half tone screen w having a line space ratio of 1: 1 is so disposed in the light tight chamber a: in relation to a glass dry plate y or strippurposes is arranged in the ordinary way, adopted with half tone Work and the dia-' phragm p or p as the case may be is cont-inuously rotated by the motor 8 in manner already described, a time exposure resulting in a blurred multiple image being photographically effected of the rotating patch v or aperture o as the case may be in the light sensitive material.

Upon completion of the exposure the image is developed in the ordinary way and if the patch '0 has been used said image Willresemble the representation shown in Fi ure 1 or if an aperture '0 has been used sald image will resemble the representation shown in Figure 3 the proportions in either case being defined by the line screen used during exposure and the variation in relative density of each small area being dependent upon the shape of the patch 1) or the aperture 12 To control the rate of change of density across each of the areas a or a the circumferential bands of p or p are varied in accordance with results obtained. For instance if a regular rate of change is required a basic chart is prepared having say twentyfive spaces; one of said spaces 1s pure white and one of said spaces is intense black the remaining spaces bein grey tones of such different and interme iate values that the chart represents twenty-four progressive tone steps. Such a chart is photographed through the improved screen and the result ant dot image is examined and'the dots in different parts of the'chart representation measured so as to see if the change in dot area in the printed result will be comparable with the change in tone of the basic chart any inaccuracies being corrected by extending or reducing the length of one or other of the circumferential bands.

In use such a screen as illustrated in Fig ure 1, may be supported by a film of celluloid or the like about five or ten one ten thousandths part of an inch in thickness said film and screen being interposed between and in close contact with the continuous tone negative transparency of the picture to be reproduced and a sheet of metal or other material having a photographically sensitive coating thereon, the actual screen surface being in contact with the photographically sharply defined dots the size of each of which is dependent upon the value of the tone at that particular part of the picture.

In some cases in order to facilitate the photographic transparency at one part of the handling of the screen when mounted on the area to relative photographic opaclty at anfilm of celluloid the flexible screen as a whole other part of the area.

may be mounted in a frame or other device adapted to hold the said screen perfectly fiat.

I claim 1. Improved means for producing screens of the type specified comprising a light tight chamber; a half tone or ruled screen in half tone relation to a film of photographically sensitive material arranged behind said screen in said chamber; a. fixed diaphragm in said chamber having an aperture in half tone relation to said screen, said aperture being similar in shape to the shape of the openings formed by the rulings of the screen employed a rotatable diaphragm associated Withsaid aperture, the opaque part of said diaphragm being of such shape as will give a predetermined rate of change of opacity in the images thereof on the sensitive material: a source of illumination and means for continuously r0- tating said diaphragm.

2. Improved means for producing screens of the type specified as claimed in claim 1, characterized by that the opaque part of the rotatable diaphragm is composed of a plurality of circumferential bands. the circumferential length of each of "which is such as will give the desired rate of change of density across each of the areas projected on" to the photographically sensitive material.

3. A method of producing a photographic screen which comprises converting a substantially, uniform beam of light into a beam.

wherein the intensity varies progressively across its area-thereafter resolving said beam into a plurality of similar beams and proiecting said beams upon a light sensitive medium.

4. Apparatus for producing a photographic screen comprising means for providing a beam of light, means for varying the intensity of said beam progressively across its area and means for resolving said beam into a plurality of similar beams all of said beams be ing positioned for efl'ecting'the projection of said beams upon a light sensitive medium.

5. Apparatus for producing a photographic screen comprising means for providing a beam of light, a diaphragm adapted for varying the intensity of said beam progressively from its center to its circumference and a screen adapted for resolving said beam into a plurality of similar beams, all of said elements being positioned for effecting the projection of said beams upon a light sensitive medium.

6. A screen for photographic purposes, photomechanical printing processes and the like, comprising a translucent sheet provided with a plurality of relatively small contiguone areas, the density of each of which uniformly and continuously varies from relative- 

